objects remain unserializable after reset_index

[bjarketol]

What happened?

With the 2024.1 release, I am unable to write objects to netCDF after having stacked dimensions with .stack() and called .reset_index() to get rid of the multi-index

What did you expect to happen?

No response

Minimal Complete Verifiable Example

import numpy as np 
import xarray as xr 
da = xr.DataArray(np.zeros([2, 3]), dims=["x", "y"])
da = da.stack(point=("x", "y"))
da = da.reset_index("point")
da.to_netcdf("test.nc")

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

86 def ensure_not_multiindex(var: Variable, name: T_Name = None) -> None:
     87     if isinstance(var._data, indexing.PandasMultiIndexingAdapter):
---> 88         raise NotImplementedError(
     89             f"variable {name!r} is a MultiIndex, which cannot yet be "
     90             "serialized. Instead, either use reset_index() "
     91             "to convert MultiIndex levels into coordinate variables instead "
     92             "or use https://cf-xarray.readthedocs.io/en/latest/coding.html."
     93         )

NotImplementedError: variable 'x' is a MultiIndex, which cannot yet be serialized. Instead, either use reset_index() to convert MultiIndex levels into coordinate variables instead or use https://cf-xarray.readthedocs.io/en/latest/coding.html.

Anything else we need to know?

Creating the stacked object from scratch and saving it to netCDF works fine. The difference is that type(da.x.variable._data) is xarray.core.indexing.PandasMultiIndexingAdapter if it was stacked and reset and numpy.ndarray if it's created from scratch

Environment

INSTALLED VERSIONS

commit: None
python: 3.11.7 | packaged by conda-forge | (main, Dec 23 2023, 14:43:09) [GCC 12.3.0]
python-bits: 64
OS: Linux
OS-release: 5.15.133.1-microsoft-standard-WSL2
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.14.3
libnetcdf: 4.9.2

xarray: 2024.1.0
pandas: 2.1.4
numpy: 1.26.3
scipy: 1.11.4
netCDF4: 1.6.5
pydap: None
h5netcdf: 1.2.0
h5py: 3.10.0
Nio: None
zarr: 2.16.1
cftime: 1.6.3
nc_time_axis: None
iris: None
bottleneck: 1.3.7
dask: 2024.1.0
distributed: None
matplotlib: 3.8.2
cartopy: 0.22.0
seaborn: 0.13.1
numbagg: None
fsspec: 2023.12.2
cupy: None
pint: 0.23
sparse: None
flox: None
numpy_groupies: None
setuptools: 69.0.3
pip: 23.3.2
conda: None
pytest: 7.4.4
mypy: None
IPython: 8.20.0

[benbovy]

Oops, my mistake in #8565 (comment).

Thanks for the report @bjarketol!